1. Field of the Invention This invention relates to a gas-balanced Brayton cycle engine and specifically to gas-balanced Brayton cycle engines designed to operate at about 150 K having input power in the range of 5 to 30 kW.
2. Background of the Invention
A Brayton-type or Brayton cycle engine includes three essential components: a gas compressor, a counter-flow heat exchanger, and an expander.
Four recent patent applications assigned to SHI Cryogenics describe gas-balanced Brayton cycle expansion engines and two adaptations, one to minimize cool down time to cryogenic temperatures, the other to cool a cryopump for pumping water vapor. A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a counterflow heat exchanger, which admits gas to an expansion space through a cold inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns the gas through the counterflow heat exchanger to the compressor.
U.S. Patent Application Publication 2011/0219810 dated Sep. 15, 2011 by R. C. Longsworth describes a reciprocating expansion engine operating on a Brayton cycle in which the piston has a drive stem at the warm end that is driven by a mechanical drive, or gas pressure that alternates between high and low pressures, and the pressure at the warm end of the piston in the area around the drive stem is essentially the same as the pressure at the cold end of the piston while the piston is moving. U.S. Patent Application Publication 2012/0085121 dated Apr. 12, 2012 by R. C. Longsworth describes the control of a reciprocating expansion engine operating on a Brayton cycle, as described in the previous application, which enables it to minimize the time to cool a mass to cryogenic temperatures. U.S. Ser. No. 13/106,218 dated May 12, 2011 by S. Dunn, et al., describes alternate means of actuating the expander piston. U.S. Ser. No. 61/504,810 dated Jul. 6, 2011 by R. C. Longsworth describes the application of a Brayton cycle engine to cooling coils for cryopumping water vapor. The engines described in published patent application 2011/0219810 and U.S. Ser. No. 13/106,218 are referred to as “Gas-balanced Brayton cycle engines”. A compressor system that can be used to supply gas to these engines is described in U.S. Patent Application Publication 2007/0253854 titled “Compressor With Oil Bypass” by S. Dunn filed on Apr. 28, 2006. The engine of this present invention incorporates a cold rotary valve which has some features in common with U.S. Pat. No. 3,205,668 dated Sep. 14, 1965 by W. E. Gifford, and U.S. Pat. No. 4,987,743 dated Jan. 29, 1991 by A. J. Lobb. It also incorporates a vibration absorbing double bumper as described in U.S. Pat. No. 6,256,997 dated Jul. 10, 2001 by R. C. Longsworth and an anti-abrasion coating on the piston as described in U.S. Pat. No. 5,590,533 dated Jan. 7, 1997 by H. Asami et al.
A cryopump for pumping water vapor requires a cryopanel that is cooled to a temperature between 120 K and 170 K. This is a lot warmer than the temperature range of 10 K to 20 K needed to cryopump air. A paper by C. B. Hood, et al., titled “Helium Refrigerators for Operation in the 10-30 K Range” in Advances in Cryogenic Engineering, Vol. 9, Plenum Press, New York (1964), pp 496-506, describes a large Brayton cycle refrigerator having a reciprocating expansion engine capable of producing more than 1.0 kW of refrigeration at 20 K. This refrigerator was developed to cryopump air in a large space chamber. Starting in the early 1970's cryopumping water vapor at temperatures in the range of 120 K to 170 K and capacities of 500 to 3,000 W have been dominated by refrigerators that use mixed gases as described in U.S. Pat. No. 3,768,273 dated Oct. 30, 1973 by Missimer. A more recent patent, U.S. Pat. No. 6,574,978 dated Jun. 10, 2003 by Flynn, et al., describes means of controlling the rate of cooling and heating a refrigerator of this type which produces about 500 to 3,000 W at about 150 K to pump water vapor
The refrigerants used in mixed gas refrigerators include some that are being phased out because of their impact on global warming. It is thus desirable to use a Brayton cycle engine which uses helium, argon, or nitrogen, all environmentally friendly. The present invention is based on the recognition that a Brayton cycle engine that operates at about 150 K can be a lot simpler than one that is designed for lower temperatures. These simplifications make it practical to design an engine that can produce over 3,000 W of refrigeration and thus compete with present mixed gas refrigerators.